Degenerate multi-photon properties of spirofluorene derivatives

Article abstract of DOI:10.1166/jnn.2010.2975

Two- and three-photon absorption properties of the fluorene-based chromophores have been investigated. The two- and three-photon absorption cross-section are found to be increased with the strength of the electron donor groups in the order of N-ethylcarbazoyl (1), triphenylamino (2), and N,N-dibutylanilino (3) groups. This nonlinear absorption enhancement can be interpreted by the increase of intramolecular charge transfer facilitated by strong electron donors and the decreased detuning energy (ΔE). Furthermore, direct laser microfabrication by two-photon photopolymerization with compound 2 as a two-photon sensitizer was carried out. Laser exposure time-dependent lateral voxel size has also been studied. Copyright

Full text of DOI:10.1166/jnn.2010.2975

Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6958–6961, 2010
Degenerate Multi-Photon Properties of
Spirofluorene Derivatives
Namchul Cho1, Patrice L. Baldeck2, Guang S. He3, Do-Hoon Hwang4, Paras N. Prasad3,
Prém Prabhakaran1, Tae-Dong Kim1, and Kwang-Sup Lee1ꢀ∗
1Department of Advanced Materials, Hannam University, Daejeon 305-811, Korea
2Laboratoire de Spectrométrie Physique, Université Joseph Fourier, CNRS (UMR 5588), France
3Institute for Lasers, Photonics and Biophotonics, Department of Chemistry, State University of New York at Buffalo, New York, USA
4Department of Applied Chemistry, Kumoh National Institute of Technology, Gumi 730-701, Korea
Two- and three-photon absorption properties of the fluorene-based chromophores have been inves-
tigated. The two- and three-photon absorption cross-section are found to be increased with the
strength of the electron donor groups in the order of N-ethylcarbazoyl (1), triphenylamino (2), and
N,N-dibutylanilino (3) groups. This nonlinear absorption enhancement can be interpreted by the
increase of intramolecular charge transfer facilitated by strong electron donors and the decreased
detuning energy (ꢀE). Furthermore, direct laser microfabrication by two-photon photopolymeriza-
tion with compound 2 as a two-photon sensitizer was carried out. Laser exposure time-dependent
lateral voxel size has also been studied.
Keywords: Two-Photon Absorption, Three-Photon Absorption, Fluorene Derivatives, 3D
Delivered by Ingenta to: McMaster University
Microfabrication.
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Copyright: American Scientific Publishers
1. INTRODUCTION
and 3PA properties of a wide range of chromophores.
While 2PA has been well studied the latter have not been
as deeply explored. There have been few studies involv-
ing application of three-photon absorbing chromophores as
gain media in stimulated emissions, and also as a medium
for optical power limiting.2ꢁ7ꢁ8 More recently its has been
proposed that a greater understanding of three-photon
absorption phenomenon would go a long way towards
developing high resolution imaging techniques utilizing IR
and NIR wavelengths.9 In the presented work we investi-
gate 2PA and 3PA properties of chromophores consisting
of the fluorene as a ꢀ-center with symmetrically substi-
tuted electron donors. We measured 2PA and 3PA cross
section (ꢂ2 and ꢂ3, respectively) by direct nonlinear trans-
mission method. It was found that the donor strength and
the detuning energy play an important role in the two- and
three-photon transition. We have also successfully demon-
strated the use of the chromophores as two-photon photo-
sensitizers in two-photon stereolithography.
Over the past decade, two- and three-photon absorption
(2PA and 3PA) behaviors from organic molecules have
generated tremendous interest because of their potential
applications in a 3D microfabrication, fluorescence imag-
ing, photodynamic therapy, and optical power limiting.1–4
For the practical applications, efficient chromophores hav-
ing a large 2PA and 3PA cross-section are required.
Therefore, numerous efforts have been made to study the
structure–property relationships of multi-photon absorbing
chromophores. It has been found that 2PA and 3PA proper-
ties are related to the strength of substituted electron donor
and acceptor and the efficiency of charge transfer through
the entire molecular backbone. In our previous studies,
we have investigated the 2PA properties of fluorene-based
chromophores and have demonstrated the 3D microfabrica-
tion using these chromophores as two-photon sensitizers.4–6
The rigid planar structure of the fluorene group allows effi-
cient symmetric intramolecular charge transfer from the
electron donor to the fluorene center in the donor-ꢀ-donor
(D-ꢀ-D) structure and, consequently, enhances the multi-
photon properties of the molecules.
2. EXPERIMENTAL DETAILS
2.1. Materials
With the development of high repetition rate lasers oper-
ating at longer wavelengths, it is feasible to study 2PA
All materials are purchased from Aldrich and used
without any further purification. Vinyl-substituted
N-ethylcarbazole, triphenylamine, and N,N-dibutylaniline
∗Author to whom correspondence should be addressed.
6958
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6958/004
doi:10.1166/jnn.2010.2975

Cho et al.
Degenerate Multi-Photon Properties of Spirofluorene Derivatives
were synthesized according to previously reported
methods.
3. RESULTS AND DISCUSSION
Scheme 1 shows a synthetic route of fluorene-based
chromophores (Compound 1, 2, and 3) for comparing
multi-photon properties. Synthesized vinyl compounds
were coupled with 9,9-bis(4-octyloxyphenyl)-2,7-dibromo-
fluorene by Heck reaction. The fluorene unit at the
9-position possesses bulky alkoxyphenyl substituents
which can increase solubility, and restrict aggregation of
molecules. All compounds were adequately characterized
2.1.1. Synthetic Procedure for Compound 1, 2 and 3
9,9-Bis(4-octyloxyphenyl)-2,7-dibromofluorene (12 mmol),
vinyl-substituted compound (30 mmol), palladium acetate
(0.08 mmol), tri-o-tolylphosphine (0.08 mmol), and trib-
utylamine (20 mL) in 100 mL of dimethylforamide
were refluxed for 24 h. After completion of the reac-
tion, the solution was concentrated and poured into
methanol to give a yellowish crude product. The pre-
cipitate was purified by a column chromatography using
n-hexane/methylene chloride (4:1 in vol.) as an eluent.
The pure compound was then obtained by recrystallization
with methanol and methylene chloride.
1
by H NMR, MALDI-TOF MS, UV/vis absorption, and
fluorescence data.
Figure 1 shows the one-photon absorption and fluores-
cence spectra for compound 1, 2, and 3. Their absorp-
tion and emission are clearly red-shifted from 399 nm to
417 nm and from 440 nm to 494 nm with the increase of
the strength of electron donors; N-ethylcarbazole < triph-
enylamine < N,N-dibutylaniline. This red-shift results
from the stabilization of the one-photon-allowed 1Bu state
by the type of amino groups. The fluorescence quan-
tum yields (ꢅf ꢃ for 1, 2, and 3 decrease with increasing
donor strength, which might be originated from nonradia-
tive decay rate (knrad) (Table I). We have also observed
these phenomena in similarly substituted spirofluorene
compounds.9
The 2PA and the 3PA properties were measured by
direct nonlinear transmission method in THF solution with
concentration of 7.8 mM/L at 775 and 1310 nm as the
probing wavelengths for the 2PA and the 3PA, respectively.
AF 350 was used as a reference solution.10 According to
the nonlinear absorption theory, the intensity change of
the incident laser beam along the optical propagation path
z can be described as:5
2.1.1.1. Compound 1. 1H NMR (300 MHz, CDCl3ꢃ:
ꢄ = 8.13 (2H, s), 8.08 (2H, d), 7.81 (2H, d), 7.55–6.93
(24H, m), 6.73 (4H, s), 4.35 (4H, q), 4.11 (4H, t), 1.89
(4H, m), 1.78–1.11 (20H, m), 0.99 (6H, t), 0.89 (6H, t);
MS (EI): m/z 1012.6 (M+1ꢃ+.
2.1.1.2. Compound 2. 1H NMR (300 MHz, CDCl3ꢃ:
ꢄ = 7.68 (2H, d), 7.47 (4H, d), 7.34 (4H, d), 7.29–6.96
(32H, m), 6.76 (4H, d), 3.88 (4H, t), 1.73 (4H, m), 1.46–
1.18 (20H, m), 0.86 (6H, t); MS (EI): m/z 1113.6 (M+1ꢃ+.
2.1.1.3. Compound 3. 1H NMR (300 MHz, CDCl3ꢃ:
ꢄ = 7.67 (2H, d), 7.46 (4H, d), 7.36 (4H, d), 7.21–7.00
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(6H, m), 6.87–6.75 (6H, m), 6.62 (4H, d), 3.91 (4H, t),
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3.29 (8H, t), 1.75 (4H, m), 1.59 (8H, m), 1.49–1.21
Copyright: American Scientific Publishers
(28H, m), 0.98 (12H, t), 0.89 (6H, t); MS (EI): m/z 1032.7
(M+1ꢃ+.
2.2. Spectroscopic Measurements
dIꢆzꢃ
UV/Vis absorption and fluorescence spectra were
recorded on a Shimadzu 310pc spectrophotometer and a
Horiba/Jobin-Yvon spectrofluorometer (SPEX 270M) in a
THF solution (1×10−5 M for all compounds). Coumarin
102 (Aldrich, 99%, ꢅf = 0.93 in ethanol) was used as
a reference for the fluorescence quantum yields. 2PA
and 3PA cross-section were measured by direct nonlinear
transmission method (775 nm and 160 fs laser pulses for
2PA and 1310 nm and 160 fs laser pulses for 3PA) in THF
solution.
= −ꢇIꢆzꢃ−ꢈI2ꢆzꢃ−ꢉI2ꢆzꢃ−···
(1)
dz
Here Iꢆzꢃ is the beam intensity through the absorbing
medium, and ꢇ, ꢈ, and ꢉ are the one-, two- and three-
photon absorption coefficients of the absorbing medium.
If the linear absorption is negligible at the excitation fre-
quency (v) and a degenerate 2PA or 3PA are the major
mechanism for the nonlinear absorption, the solutions of
Eq. (1) for the degenerate 2PA process is:
Iꢆ0ꢁvꢃ
Iꢆzꢁvꢃ =
(2)
2.3. Two-Photon Microfabrication
1+ꢈꢆvꢃzIꢆ0ꢁvꢃ
A titanium sapphire laser mode-locked at 80 MHz and a
780 nm wavelength with pulses of less than 100 fs were
utilized as the light source for two-photon based micro-
fabrication. A set of two galvano mirrors was used to
move the focused laser beam in the horizontal plane, and
a piezoelectric stage was used for the vertical alignment
of the beam. 0.1 wt% of compound 2 was mixed with
a photo-curable acrylic resin (SCR-500). The laser beam
was closely focused on the volume of the resin through a
microscope with a high numerical aperture lens.
Pd(OAc)2
R
R
P(o-tolyl)3
Br
Br
Et3N
+
R
DMF
H3C(H2C)7O
O(CH2)7CH3
H3C(H2C)7O
O(CH2)7CH3
N
R:
*
N
*
N
,
,
*
1
2
3
Scheme 1. Synthetic route for compound 1, 2, and 3.
J. Nanosci. Nanotechnol. 10, 6958–6961, 2010
6959

Degenerate Multi-Photon Properties of Spirofluorene Derivatives
Cho et al.
Table II. Two- and three-photon photophysical properties.
(a)
1
1.0
Compd
ꢂ2 (10−20cm4/GW)
ꢂ2 (GM)a
ꢂ3 (10−25cm6/GW2)
2
3
AF350
0.530
0.167
0.500
0.602
132
42
124
150
2ꢏ64
1ꢏ45
1ꢏ84
1ꢏ84
0.8
1
2
3
0.6
0.4
0.2
0.0
2PA and 3PA cross section were measured by direct nonlinear transmission method
(775 nm and 160 fs laser pulses for 2PA and 1310 nm and 160 fs laser pulses for
3PA) in THF solution. Molar concentration is 5 mM/L for AF350 and 7.8 mM/L for
1–3. a2PA cross section (1 GM ≡ 10−50 cm4s photon−1 molecule−1). Experimental
uncertainty: 15%.
Ti, ꢈ and ꢉ can be experimentally determined. Using these
values and molar concentration, the 2PA and 3PA cross-
section (ꢂ2 and ꢂ3) can be determined from the following
350
400
450
500
Wavelength (nm)
two equations:
(b)
ꢈ
ꢂ2 =
ꢂ2 =
(4)
10−3NAd0
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
and
ꢉ
(5)
10−3NAd0
where NA is the Avogadro constant and d0 is the molar
concentration of the molecules. 2PA and 3PA cross-section
values for 1, 2, and 3 are shown in Table II. The ꢂ2 value
for 3 is 150 GM, which is 3.6 times larger than that for
1 (42 GM).
It is well known that strong electron donor facilitates
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the intramolecular charge transfer over the entire molecule
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and decreases the detuning energy (ꢀEꢃ (see Table I)
Copyright: American Scientific Publishers
400
450
500
550
600
and, consequently enhances the 2PA as well as the 3PA
activity.11 Therefore enhanced ꢂ2 value of 3 results from
the increase of the strength of electron donor and the
decrease of the ꢌE. Furthermore, this nonlinear enhance-
ment was also observed in the 3PA process. The ꢂ3
value for 1 was 1.45 × 10−25 cm6/GW2, and the val-
ues for 2 and 3 were 1.84 × 10−25 cm6/GW2. Due to
their strong multiphoton absorption properties the flourene
Wavelength (nm)
Fig. 1. (a) One-photon absorption and (b) fluorescence spectra for com-
pound 1, 2, and 3.
and for the degenerate 3PA process is:
Iꢆ0ꢁvꢃ
1+2ꢉꢆvꢃzI2ꢆ0ꢁvꢃ
where Iꢆ0ꢁvꢃ is the incident intensity of the excitation
light, and ꢈꢆvꢃ and ꢉꢆvꢃ are the 2PA and 3PA coefficients
respectively. From the value of the input intensity Iꢆ0ꢁvꢃ
and the value of the corresponding nonlinear transmissivity
Iꢆzꢁvꢃ = ꢀ
(3)
400 nm
Table I. One- and two-photon photophysical properties.
ꢊaabs
ꢋaflu
Egee
Eefe
ꢌEg
ꢀ
Compd (nm)
ꢍb
(nm)
ꢌꢎc
ꢅfd
(eV) (eV) (eV)
1
2
3
399
411
417
12.8
11.7
10.1
440
459
494
2335 0.86
2544 0.71
3738 0.60
3.11
3.02
2.97
—
3.3
3.3
—
1.36
1.32
aAbsorption and emission maxima measured in THF; bMolar extinction coefficient
(104 M−1 cm−1ꢃ; cStokes shift; dFluorescence quantum yield; eEnergy of the tran-
sitions between the ground state and the one-photon allowed excited state; f Energy
of the transitions between the ground state and the two-photon allowed excited
state obtained from the two-photon induced fluorescence excitation experiment;
Fig. 2. SEM image of fabricated voxels by two-photon initiated pho-
topolymerization. The size of voxel was varied by changing exposure
time in the arrow direction (2, 4, 8, 16, 32, and 64 ms). The inset
shows the magnified image of voxels, which reaches 200 nm for lateral
dimension.
gDetuning energy ꢌE = Ege −1/2Ege
.
ꢀ
6960
J. Nanosci. Nanotechnol. 10, 6958–6961, 2010

Cho et al.
Degenerate Multi-Photon Properties of Spirofluorene Derivatives
600
550
500
450
400
350
300
250
been said above, that 3 can be used effectively as an effi-
cient two-photon sensitizer because of its large ꢂ2 value,
which is comparable with the best reported spirofluorene
derivatives.4
2 ms
4 ms
8 ms
16 ms
32 ms
64 ms
4. CONCLUSION
In conclusion, we have synthesized D-ꢀ-D type spiro-
fluorene derivatives, and investigated its structure–property
relationship. 2PA and 3PA cross-sections for the com-
pound 1, 2, and 3 increased with the strength of elec-
tron donors, which result from increased intramolecular
charge transfer and decreased detuning energy (ꢌEꢃ. We
have also studied the two photon polymerization process
in order to make a 3D microfabrication. Fabricated voxel
size was successfully controlled by exposure time. The
resolution of voxel reaches 250 nm, which is difficult to
obtain using conventional one-photon based laser rapid
prototyping. These results are important not only for 3D
optical data storage application in the view of basic phys-
ical knowledge but also a basis of structure–property rela-
tionship on 2PA and 3PA study.
0
10
20
30
40
50
60
70
80
Exposure time (ms)
Fig. 3. Variation of voxel size by changing exposure time.
based ‘D-ꢀ-D’ molecules studied here has potential in
practical applications.
We used compound 3 to study the exposure time depen-
dent voxel size for two-photon initiated microfabrication
(or photopolymerization). A voxel is a volume pixel that
forms the basic building unit of a microstructure which
can be three dimensionally repeated to fabricate complex
structures.12 In the two-photon microfabrication, a deep
understanding of the single voxel is crucial to achieve
2D and 3D microfabrication with high resolution. Gen-
erally, the voxel size can be controlled by adjusting the
laser power or by changing the laser exposure time. An
efficient two-photon dye can facilitate the fabrication of
structures at lower powers due to its high photo sensiti-
zation capability. This enables creation of fine structures
at low energies. The interaction between the sensitizer
and light is dependent on the square of light intensity,
which enables sub-micron spatial resolution because two-
photon initiated photopolymerization occurs mainly at a
focal point of incident laser beam in the resin. Figure 2
shows a scanning electron microscope image (SEM) of
fabricated voxels with 3 in a urethane-acrylate photopoly-
merizable resin (SCR 500, JSR Co). The size of voxel
was varied by changing exposure time at constant power
of 15 mW in the arrow direction (2, 4, 8, 16, 32, and
64 ms). The reproducibility of the voxel dimensions were
tested by zigzag repetition of the microfabrication. The
inset in Figure 2 shows the magnified image of voxels. As
shown in the inset, we successfully fabricated high res-
olution of microstructures to below 250 nm using short
exposure time and weak laser power. This resolution is
better than that achieved in our previous results using a
long exposure technique.4
Acknowledgment: This work was supported by the
National Research Foundation of Korea (NRF) grant from
the Korean Government (MEST) (NRF-2009-0063224 and
Delivered by Ingenta to: McMaster University
M10503000217-0505M0300-21700). Kwang-Sup LEE
IP: 5.62.154.195 On: Wed, 15 Jun 2016 15:55:13
acknowledges supports from University Research Program
Copyright: American Scientific Publishers
in Hannam University (2010).
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Received: 31 December 2008. Accepted: 1 November 2009.
J. Nanosci. Nanotechnol. 10, 6958–6961, 2010
6961